As a transmission network for optical communication, single-mode optical fibers having zero dispersion in the wavelength band of 1.3 .mu.m are installed all over the world. In recent years, as the information-oriented society has developed, the quantity of communication information has been dramatically increasing. The explosion of the quantity of information has made wavelength division multiplex transmission (WDM transmission) widely accepted in the field of communication. Now, the world is in the age of wavelength division multiplex transmission. Wavelength division multiplex transmission is a method in which a wavelength for optical communication is divided into a plurality of wavelengths to transmit a plurality of optical signals instead of using only one wavelength for optical communication, and therefore, is a light transmission method which is suitable to large-capacity high-speed communication.
However, in general, the installed single-mode transmission optical fibers which have zero dispersion at 1.31 .mu.m have a large transmission loss. Hence, when wavelength division multiplex optical communication is to be realized using the wavelength band of 1.3 .mu.m, since the wavelength range of the installed single-mode transmission optical fibers does not match a wavelength band of 1.55 .mu.m which is a gain bandwidth of a regular light amplifier, the light amplifier can not be used and long-distance optical communication is troubled accordingly. Due to this, in recent years, wavelength division multiplex optical communication is conducted in the wavelength band of 1.55 .mu.m using the installed single-mode transmission optical fibers which have zero dispersion in the wavelength band of 1.3 .mu.m.
However, when optical communication is conducted in the wavelength band of 1.55 .mu.m using a single-mode transmission optical fiber which has zero dispersion at 1.31 .mu.m, since the installed single-mode transmission optical fibers have positive dispersion and a positive dispersion slope in the wavelength band of 1.55 .mu.m, as optical signals propagate through the single-mode transmission optical fiber, the signal at each one of the wavelength division multiplexed wavelengths disperse largely. This makes it difficult to separate the signals at a receiving end, degrades the quality of the optical communication, and impairs the reliability of the optical communication.
To solve such problems, dispersion compensating optical fibers have been developed recently. A dispersion compensating optical fiber has negative dispersion. If a dispersion compensating optical fiber is connected to a receiving end of a single-mode transmission optical fiber, positive dispersion of optical signals which propagate through the single-mode transmission optical fiber is reduced by the negative dispersion of the dispersion compensating optical fiber so that the optical signals are received at the receiving end as they have substantially zero dispersion. As a dispersion compensating optical fiber is connected to a single-mode transmission optical fiber in this manner, it is possible to separate the respective wavelength division multiplexed optical signals at a receiving end, and therefore, a dispersion compensating optical fiber is expected to realize large-capacity high-speed communication of a high quality.
This type of a dispersion compensating optical fiber is formed as a module. It is a common practice to connect a dispersion compensating optical fiber which is formed as short as possible to a receiving end of a single-mode transmission optical fiber for the purpose of compensation of dispersion.